The present invention pertains to processes for the separation of potassium salts of perfluoroalkanoic acids having at least 1 and at most 12 carbon atoms per molecule from aqueous solutions thereof.
As used herein the term "perfluoroalkanoic acids" refers to acyclic monocarboxylic acids all or most of the hydrogen atoms of which have been replaced by fluorine, such as, for example, trifluoroacetic, heptafluorobutyric, perfluorooctanoic, and similar acids.
Heretofore alkali-metal salts of perfluoroalkanoic acids such as are formed by hydrolysis of esters, nitriles, and acyl fluorides of such acids in aqueous alkaline media have been recovered from such aqueous solutions by evaporating the said solutions to dryness and extracting the dry residue with ethanol. This process has the disadvantage that the extraction must be performed in the absence of water with absolute (anhydrous) ethanol since inorganic salts readily dissolve in aqueous ethanol.
In another known process, the perfluoroalkanoic acid is extracted from an aqueous acidic medium with diethyl ether. This process has the disadvantage that it is not adapted for the separation of lower perfluoroalkanoic acids since these lower acids form hydrates which cannot be separated or can be separated only with difficulty by distillation from diethyl ether and other organic solvents or by other methods, such as by drying over phosphorus pentoxide. The amount of water that is carried over in distillation can be very high. Heptafluorobutyric acid, for example, forms an azeotrope which contains 72% by weight of water and only 28% by weight of the acid.
In the case of homologous mixtures of perfluoroalkanoic acids, but not perfluoroalkanoic acids of uniform chain length, it is moreover a condition of an extraction process that all perfluoroalkanoic acids or their salts have essentially the same solubility in the extractant, regardless of their chain length. The solubility in water of potassium salts of perfluoroalkanoic acids, for example, increases substantially with decreasing chain length so that as a rule liquid-liquid extraction with an organic solvent of these salts from an aqueous solution thereof can only be accomplished with difficulty.
Furthermore, processes have also been described for the extraction of individual perfluoroalkanoic acids. Besides the perfluoroalkanoic acids that are produced, inorganic fluorides or other salts are formed therewith so that a complete separation of the foreign salts from the desired salt of the perfluoroalkanoic acid is not achieved.
In the case of mixtures of inorganic salts and salts of perfluoroalkanoic acids with a wider range of chain lengths, the extraction of the dry residue of the evaporated solution does not produce a complete separation since the inorganic salts are also partially soluble in the extractants.
On the other hand, to separate free perfluoroalkanoic acids that are contaminated with accompanying inorganic acids, either by extraction or distillation, leads ultimately to corrosion of the extraction or distillation apparatus.